Very unlikely anyone would give up Mugen power battery after they have used to use it and know the benefits.

But if you can get just the back cover, making bigger 2xJapod 2600 mAh battery is not that hard if you just have solder machine available. The most simplest way, is not to disassemble neither batteries, but just solder (+) and (-) cables to the connectors of "the lower" battery in that way it is still connectible to the battery socket of N900. Maybe also use small drill to make space, remove plastic on top of the connectors for the cables to come up from the battery cave.

The above thread about battery mod is still missing instructions of this most simple way of doing double capacity battery. What I did was not to remove any plastic, because it worked without also, but using just little thicker cables it would make sense to remove the black plastic on those places where black and red cables come up (see the pic on the previous link). One must not use too thin cables, because N900 will charge at max 600 mA (and even @1200 mA in a worst case) each battery.